This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Geologists did a lot of talking to trees in the aftermath of Mount St. Helens's eruption. They had a lot of questions, and the trees had a lot of answers*.
Few talked to the trees as extensively as Richard Waitt. He was investigating the blast deposits, and found the trees to be quite helpful. His work in the field led him to identify three primary layers: A1, the base, was pretty full of gravel. A2, the next level up, was a coarse sand, and the final, A3, a fine air-fall sand.** Throughout those layers were trees and bits thereof, and he queried them closely to figure out the progress of the lateral blast and how it had left its deposits.
Near the volcano, at distances around 6-12 kilometers (3.7 - 6.2 miles) from the vent, he found downed trees either fully buried by the gravelly bottom layer, or protruding at an angle from it. Those trees retained some of their bark even on the sides that had taken the direct brunt of the blast before they fell, unlike the stumps and standing trees that had been stripped completely. Since there was no blast deposit beneath them, those fallen trees must have come to rest either just before or during those moments the gravel was rattling down. The fact it hadn't scoured the bark away as it landed told him that layer A1 wasn't A+ at abrading horizontal surfaces. Both layers A1 and A2 had trouble achieving more than a thin layer atop the logs, due to curvature, and the lateral nature of the blast.
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Even those thinnish deposits were somewhat protective. The bits of tree that stuck out from them fared rather worse than the buried bits: they lost their bark, and got a good scorching in the bargain. Scorching didn't happen on parts already coated by A1, which shows A1 wasn't such hot stuff. The scorching showed that "a flux of very hot gas" had blasted out from the volcano just after A1's arrival. After all the baking and abrading happened during A1 and A2's deposition, layer A3 dusted the results, showing it got there last.
Nothing in that sector survived. Rick was talking to dead trees. But when he moved on 10-12 kilometers (6.2 - 7.5 miles) north, he discovered something remarkable: life. The trees that had remained standing throughout the blast were perished, stripped naked of needles and badly scorched. But some had flexed, bent, and been buried by the blast deposits, much like they might be covered by an avalanche. When water began cutting gullies in the deposits a few days after the eruptions, the trees were freed of their tomb and whipped back upright, all green and lovely, if a bit put-upon. Even the delicate mosses on resident rocks survived, so long as they were decently covered, then quickly disinterred. These green, growing things supported the testimony of their dead relatives: that blast of searing-hot gas had certainly arrived only after the erosive front and layer A1. And that layer had provided the insulation necessary for some trees to survive.
Not that survival was simple, or even likely.
Further from the volcano, at around 15-25 km (9.3 - 15.5 miles), the blast had lost enough energy to stop carrying A1, but the erosive edge was still powerful enough to knock down some trees and turn others into snags. The fallen trees, buried beneath a nice layer of A2, held on to their bark. The snags weren't so lucky: they lost the bark on sides facing the volcano, though only a meter or two above ground and up. The hot flux had arrived with layer A2 and got busy stripping and singing, but lagged the erosive edge. On the far end of the downed timber zone, the standing trees lost their needles and had their twig ends scoured by the blast, but their lower branches, coated by A2, retained dead, scorched needles. A2 couldn't protect them from the hot gas it arrived with and was outlasted by, but did provide some defense.
At the scorch zone, the layer of A2 became thin indeed. But the flow hadn't lost its heat, hence the scorching. A coat of silty A3 plastered some of the seared branches, which were just as toasted as the bare ones. The hot flux, then, got there and left before A3 arrived.
Those were the tales of trees covered by the deposits. Next, we'll interview those trees that actually became deposits themselves.
*No illicit substances were involved. The trees didn't actually talk. Everyone's fine.
**Not beach sand - sand refers here to the size of the rock particles. This is sand-sized ash.
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References:
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.